item.categories are not optional

[jaredkotoff]

My feed items do not have categories, and the docs list this as optional.

When running feed.xml():

TypeError: undefined is not an object (evaluating 'item.categories.forEach')
      at /node_modules/rss/lib/index.js:71:9
      at forEach (:1:21)
      at generateXML (/node_modules/rss/lib/index.js:63:5)
      at /node_modules/rss/lib/index.js:185:17

When I change node_modules/rss/lib/index.js:71 to:

item.categories?.forEach(function(category) {
            ifTruePush(category, item_values, { category: { _cdata: category } });
        });

I am able to generate the xml feed